As technologies further advance, wireless power transfer (WPT) has emerged as an efficient and convenient mechanism for powering or charging mobile devices such as mobile phones, tablet PCs, digital cameras, MP3 players and/or the like. A wireless power transfer system typically comprises a primary side transmitter and one or more secondary side receivers. The primary side transmitter is magnetically coupled to the secondary side receiver(s) through a magnetic coupling. The magnetic coupling may be implemented as a loosely coupled transformer having a primary side coil formed in the primary side transmitter and a secondary side coil formed in a secondary side receiver.
FIG. 1 illustrates a block diagram of a wireless power transfer system. The wireless power transfer system 100 shown in FIG. 1 is an exemplary system recommended by the Alliance for Wireless Power (A4WP) organization. The wireless power transfer system 100 shown in FIG. 1 includes a power transmitter and a power receiver. Through magnetic coupling, power is transferred from the power transmitter to the power receiver.
The power transmitter includes a transmitter dc/dc converter, a power amplifier, an impedance matching circuit and a resonant circuit connected in cascade between a power input and a transmitter coil. The power transmitter may further comprise a transmitter Bluetooth unit having a first input/output coupled to a receiver Bluetooth unit and a second input/output coupled to the transmitter dc/dc converter of the power transmitter.
The power receiver includes a resonant circuit, a rectifier, a receiver dc/dc converter connected in cascade between a receiver coil and a load. The power receiver may further comprise the receiver Bluetooth unit having an input/output coupled to the transmitter Bluetooth unit. More receivers with the same architecture as shown in FIG. 1 may be added to the wireless power transfer system 100 to form a multiple receiver system.
According to the standard of A4WP, the power transmitter operates at a fixed system frequency within a frequency band ranging from about 6.765 MHz to about 6.795 MHz (a 6.78 MHz nominal frequency). It should be noted that the power transmitter may operate at a frequency different from the one described above. The transmitter power amplifier converts dc power at its input to high frequency ac power having a frequency within the frequency band described above. The transmitter coil, coupled to the power amplifier through a resonant circuit (usually one or more capacitors), forms a transmitter resonant tank with the resonant circuit and generates a magnetic field at the system frequency. Through magnetic coupling, power is transferred to the receiver coil nearby. Likewise, the receiver coil and the resonant circuit of the power receiver form a receiver resonant tank.
Both the resonant circuit coupled to the receiver coil and the resonant circuit coupled to the transmitter coil may comprise one or more capacitors. The resonant frequency of the transmitter resonant tank and the resonant frequency of the receiver resonant tank are designed to be at the system frequency, which is determined by the switching frequency of the power amplifier.
In order to match the power capability and electrical parameters of the power amplifier and the resonant tank in the power transmitter, an impedance matching circuit may be placed between the power amplifier and the transmitter resonant circuit as shown in FIG. 1.
The rectifier in the power receiver converts the high frequency ac power from the receiver coil into dc power and delivers the dc power to the load through the receiver dc/dc converter. In the system shown in FIG. 1, for a given input voltage Vin sent to the power amplifier, the output voltage Vo at the rectifier may vary in a wide range due to a variety of factors such as the coupling coefficient changes between the transmitter and the receiver, load changes and the like.
In order to regulate the output voltage of the receiver within an acceptable range, the transmitter dc/dc converter may be employed to control the voltage sent to the power amplifier, and the receiver dc/dc converter may be employed to further regulate the voltage fed to the load. Because the input power is most likely from an ac/dc adapter plugged into an ac source, the transmitter dc/dc converter may be implemented as a dedicated dc/dc converter coupled to the ac/dc adapter. Alternatively, the transmitter dc/dc converter may be part of an ac/dc adapter. Similarly, the receiver dc/dc converter is usually implemented as a dc/dc converter. The load can be actual loads such as integrated circuits (ICs), a battery and the like. Alternatively, the load can be a downstream converter such as a battery charger, a dc/dc converter coupled to an actual load and the like.
The transmitter Bluetooth unit and the receiver Bluetooth unit form a Bluetooth communication subsystem providing a communication channel between the power receiver and the power transmitter. For example, the voltage control signal may be communicated between the transmitter and the receiver through the Bluetooth communication subsystem.
The wireless power transfer system 100 shown in FIG. 1 includes many power processing stages. Many components in the system shown in FIG. 1 may have high voltage/current stresses. It is desirable to have a wireless power transfer system having better efficiency and lower voltage/current stresses.